This invention relates to a heat treatment apparatus for heat-treating an object to be treated, such as semiconductor wafers or others.
A conventional heat treatment apparatus of this kind is a low-pressure CVD apparatus including a heat treatment unit of, e.g., FIG. 7 (see e.g., specification of U.S. Pat. No. 5,048,800). This conventional heat treatment apparatus will be explained with reference to FIG. 7. As shown in FIG. 7, this low-pressure CVD apparatus comprises a heating furnace 20, a double-wall reaction vessel (tube) 30 of quartz housed in the heating furnace with the axis aligned with that of the heating furnace 20, and a heat treatment boat 40 for holding a plurality of wafers W to be heat-treated horizontal spaced vertically from each other, the heat treatment (wafer) boat being removably loaded into the reaction vessel 30. The heat treatment boat 40 is loaded into the reaction vessel 30 with the axis of the boat aligned with that of the vessel, and reaction gases are fed onto the surfaces of the wafers W held on the heat treatment boat 40 to form a nitrogen oxide or other films concurrently on the surface of each wafer W.
The reaction vessel 30 comprises a double-wall vessel including an outer cylinder 31 having only the lower end opened, and an inner cylinder 32 inserted in the outer cylinder 31 conically with the latter, a manifold 33 connected to the lower end of the double-wall vessel. The manifold 33 engages with the flange of the heat treatment boat 40 to seal the reaction vessel 30 to retain the interior of the reaction vessel 30 under a required low pressure.
The heat treatment boat 40 comprises a wafer holder 41 having a plurality of grooves for holding a plurality of wafers W vertically spaced from each other, and a heat insulator 70 for insulating the interior of the reaction vessel 20 from the outside to maintain the interior of the vessel 20 at a constant temperature.
This heat insulator 70 comprises a support 70A made of a plurality of support rods 70A, four mounts 70B of quartz each supported by the support at positions a little inner of the peripheral edge, vertically spaced from each other, a cylinder 70C of the same material as the mounts 70B surrounding the mounts 70B, and a seat bearing the support 70A, the mounts 70B and the cylinder 70C. The heat-insulator 70 is closed at the upper end with a disk on the lower end of the wafer holder 41, and closed at the lower end with the seat 70D.
In the conventional low-pressure CVD apparatus of FIG. 7, the mounts 70B and the cylinder 70C of the heat insulator 70 are made of opaque quartz or opaque quartz having the surface treated with sand blasts or others for the prevention of heat rays from the heat insulator 70, whereby the heat insulating efficiency is enhanced. Cooling water is caused to flow through the passage of the manifold 33 for the suppression of temperature rises of the manifold 33. But the cooling water flowing through the passage is not sufficient to cool the manifold 33 because opaque quartz of the mounts 70B cannot sufficiently repel the heat rays in the reaction vessel 30. Temperature increases thermally degrade the O-rings near the passage and gradually evaporate a magnetic fluid in the magnetic seal unit, with the result that the sealing at the respective parts is deteriorated. This has been a problem. A 200.degree.-500.degree. C. area is formed in the upper part of the heat insulator. Grown films in this area have poor adhesion. Gas flows whose velocity is near the acoustic velocity are generated around the heat insulator 70 in a 1 Torr-vacuum. These gas flows peel off the films of poor adhesion into particles, which lower yields of the heat treatment.
The conventional low-pressure CVD apparatus including an evacuated heat insulator has high heat insulation, but the evacuation portion of the hollow vessel which is the heat insulator sometimes has imperfect sealing. Air leaks through the imperfectly sealed evacuation portion to impair film forming reactions, lower the heat insulation, or cause cracks in the evacuation portion, damaging the heat insulator. This heat insulator has such good heat insulation that a temperature of the manifold 33 and the sealing flange are maintained below 100.degree.-150.degree. C. to thereby prevent the thermal degradation of the O-rings, but reaction gases, such as ammonium and dichlorosilane, etc., tend to have side reactions to generate particles, as of ammonium chloride, etc. The particles lower yields of the heat treatment, and especially in micron-processing of submicrons, such as 4MDRAM, on semiconductor wafers, these particles affect the film formation. This is also a problem.
Furthermore, thermal convection occur in the reaction vessel 30 due to a large temperature difference between the heat treatment section and the low-temperature area, and break temperature uniformity in the heat treatment section. A problem is that homogeneous films cannot be formed.